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A test-tube baby is the product of a successful human reproduction that results from methods beyond sexual intercourse between a man and a woman and instead utilizes medical intervention that manipulates both the egg and sperm cells for successful fertilization. The term was originally used to refer to the babies born from the earliest applications of artificial insemination and has now been expanded to refer to children born through the use of in vitro fertilization, the practice of fertilizing an embryo outside of a woman's body.

Homology is a central concept of comparative and evolutionary biology, referring to the presence of the same bodily parts (e.g., morphological structures) in different species. The existence of homologies is explained by common ancestry, and according to modern definitions of homology, two structures in different species are homologous if they are derived from the same structure in the common ancestor.

Sand dollars are common marine invertebrates in the phylum Echinodermata and share the same class (Echinoidea) as sea urchins. They have served as model laboratory organisms for such embryologists as Frank Rattray Lillie and Ernest Everett Just. Both Lillie and Just used Echinarachnius parma for their studies of egg cell membranes and embryo development at the Marine Biological Laboratory (MBL) at Woods Hole, Massachusetts, in the early 1900s.

Tooth enamel contains relics of its formation process, in the form of microstructures, which indicate the incremental way in which it forms. These microstructures, called cross-striations and striae of Retzius, develop as enamel-forming cells called ameloblasts, whcih cyclically deposit enamel on developing teeth in accordance with two different biological clocks. Cross-striations result from a twenty-four hour cycle, called a Circadian rhythm, in the enamel deposition process, while striae of Retzius have a longer periodicity.

Endoderm is one of the germ layers-- aggregates of cells that organize early during embryonic life and from which all organs and tissues develop. All animals, with the exception of sponges, form either two or three germ layers through a process known as gastrulation. During gastrulation, a ball of cells transforms into a two-layered embryo made of an inner layer of endoderm and an outer layer of ectoderm. In more complex organisms, like vertebrates, these two primary germ layers interact to give rise to a third germ layer, called mesoderm.

Mesoderm is one of the three germ layers, groups of cells that interact early during the embryonic life of animals and from which organs and tissues form. As organs form, a process called organogenesis, mesoderm interacts with endoderm and ectoderm to give rise to the digestive tract, the heart and skeletal muscles, red blood cells, and the tubules of the kidneys, as well as a type of connective tissue called mesenchyme. All animals that have only one plane of symmetry through the body, called bilateral symmetry, form three germ layers.